Most existing tactical systems rely, directly or indirectly, on the presence of accurate GPS data for navigation and positioning, at least in part. A mobile unit, for instance, will typically integrate GPS data with that generated by an onboard Inertial Navigation System (INS) to produce an integrated GPS/INS solution. A stationary unit may utilize GPS to establish its fixed position.
An Inertial Navigation System, such as that used on the mobile units discussed above, is a navigation aid that uses a computer, motion sensors (e.g. accelerometers) and rotation sensors (e.g. gyroscopes) to continuously calculate, via dead reckoning, the position, orientation, and velocity (direction and speed of movement) of a moving object without the need for external references, such as GPS data. Such a system generally requires information regarding the starting position, velocity and orientation to be known to provide usable data. Such INSs are also susceptible to drift, as errors in estimates compound and accumulate over time.
Global Positioning Systems, since they are not susceptible to drift errors, due to their lack of reliance on previous estimates in updating positional data, are often used in conjunction with INS to provide robust position and velocity data. GPS, however, is susceptible to experiencing signal loss or corruption due to terrain and other variables, which may result in the GPS providing inaccurate information or ceasing to function entirely in some conditions. During these periods, the INS can be used to mitigate this loss of GPS data.
A major concern by all users of GPS technology, however, is the relative susceptibility of the GPS signal to intentional interference, such as may be encountered during wartime operations. It has been demonstrated that very inexpensive and simple hardware can be effectively used to deny platforms access to the GPS signal (jamming) and an increasingly serious threat exists in the development of means to interfere in a way that makes a GPS receiver produce erroneous results (spoofing).
Many of the current methods for the detection and/or mitigation of intentional GPS interference (jamming and spoofing) are based on the signal properties and antennae technology available for enhancing GPS receivers. For example, antennae-nulling, where an antenna is adjusted such that its dead zone, or zone of reduced sensitivity, encompasses a source of interference, is one option that is used to minimize the effects of jamming. Antennae-nulling must be performed by the transmitting platform and, dependent on the location of the source(s) of interference and their position relative to the intended receiver, may not be capable of sufficiently attenuating the interference. Especially in cases where the source of interference is relatively close to the intended recipient of the signal, antennae-nulling may result in severe signal loss to the intended recipient.
A second option for the detection and/or mitigation of intentional GPS interference involves enhanced signal processing techniques, which are used to increase the anti-jam capabilities of GPS receivers. Such techniques, however, are computationally expensive. The US Government has also introduced a 3rd GPS frequency for the purpose of enabling the use of enhanced GPS receivers that would minimize their sensitivity to interference signals, this technique, however, requires receivers to be capable of receiving and processing the new frequency, resulting in integration of this technology being time consuming and expensive.
Although the problems described above were described primarily in the context of military and tactical situations, GPS interference is a problem for both military and commercial applications.
What is needed, therefore, are techniques for detecting and overcoming jamming and spoofing that do not require modification of the underlying hardware and that are not computationally expensive.